The present invention relates to performance improving additives of long chain alcohols for surfactant foaming solutions which are injected in conjunction with steam or carbon dioxide floods to improve conformance.
When an oil reservoir is subjected to steam injection, steam tends to move up in the formation, whereas condensate and oil tends to move down due to the density difference between the fluids. Gradually, a steam override condition develops, in which the injected steam sweeps the upper portion of the formation but leaves the lower portion untouched. Injected steam will tend to follow the path of least resistance from the injection well to a production well. Thus, areas of high permeability will receive more and more of the injected steam which further raises the permeability of such areas. This phenomenon exists to an even larger degree with low injection rates and thick formations. The steam override problem worsens at greater radial distances from the injection well because steam flux decreases with increasing steam zone radius.
Although residual oil saturation in the steam swept region can be as low as 10%, the average residual oil saturation in the formation remains much higher due to poor vertical conformance. Thus it is because of the creation of steam override zones that vertical conformance in steam floods is usually poor.
A similar conformance problem exists with carbon dioxide flooding. Carbon dioxide has a large tendency to channel through oil in place since carbon dioxide viscosity may be 10 to 50 times lower than the viscosity of the oil in place. This problem of channeling through oil is exacerbated by the inherent tendency of a highly mobile fluid such as carbon dioxide to preferentially flow through more permeable rock sections. These two factors, unfavorable mobility ratios between carbon dioxide and the oil in place and the tendency of carbon dioxide to take advantage of permeability variations, often make carbon dioxide flooding uneconomical- Conformance problems increase as the miscibility of the carbon dioxide with the oil in place decreases.
Although not much attention has been devoted to carbon dioxide conformance, it has long been the concern of the oil industry to improve the conformance of a steam flood by reducing the permeability of the steam swept zone by various means. The injection of numerous chemicals such as foams, foaming solutions, gelling solutions or plugging or precipitating solutions have been tried. Because of the danger of damaging the reservoir, it is considered important to have a non-permanent means of lowering permeability in the steam override zones. For this reason, certain plugging agents are deemed not acceptable. In order to successfully divert steam and improve vertical conformance, the injected chemical should be (1) stable at high steam temperatures of about 300.degree. to about 600.degree. F., (2) effective in reducing permeability in steam swept zones, (3) non-damaging to the oil reservoir and (4) economical.
The literature is replete with references to various foaming agents which are employed to lower permeability in steam swept zones. The vast majority of the foaming agents in the prior art require the injection of a non-condensable gas to generate the foam in conjunction with the injection of steam and the foaming agent. U.S. Pat. Nos. 3,410,344 and 3,994,345 disclose the use of a steam foaming agent selected from the generic groups of polyethoxyalkanols and alkylaryl sulfonates to reduce permeability in steam channels. U.S. Pat. No. 4,018,278 discloses the use of sulfonated, ethoxylated alcohols or alkylphenols in surfactant flooding solutions without the use of steam.
Copending U.S. patent application Ser. No. 896,710, filed Jun. 10, 1992, now U.S. Pat. No. 5,209,367, discloses the use of fatty acids having about 12 to about 20 carbon atoms as additives to surfactant foaming solutions used in steam or carbon dioxide floods where the ratio of fatty acid to surfactant in the foaming solution is between about 1:4 to 3:2. Copending U.S. patent application Ser. No. 964,741 filed Dec. 24, 1992, discloses the use of fatty acid salts as steam foaming agents, and not additives to foaming systems in steam floods. The aqueous solution containing the fatty acid salts must have a pH between about 8 and about 12 and a salinity greater than about 1%.